Insulating glass (IG) units, which are used in the manufacture of double glazed windows and doors, typically comprise two parallel sheets of glass held a small distance apart by a spacer bar. The small area between the two parallel sheets of glass, i.e. the cavity, is generally filled with air or an inert gas such as argon.
Conventional IG units typically use two types of sealants to, assist in adhering the glass to the spacer bar, and this type of construction is known as dual-sealed. In such dual-sealed IG units, the first type of sealant used is the innermost, or ‘primary’, sealant. This ‘primary’ sealant is used to form a seal between the spacer bar and the glass, wherein this seal is inside the cavity between the two glass sheets. Conventionally the ‘primary’ sealant is a thermoplastic sealant based on polyisobutylene, and its function is to prevent moisture vapour from entering the cavity of the IG unit and causing condensations. In the case of a gas-filled IG unit, the ‘primary’ sealant also acts as a barrier to the escape of inert gas (typically argon) from the unit. The ‘primary’ sealant has little mechanical strength and relatively poor adhesion as compared to the cured version of the second type of sealant used in dual-sealed IG units.
Considering now the second type of sealant, this is the outermost sealant and is again used to form a seal between the spacer bar and the glass, but this time the seal is not inside the cavity between the two glass sheets but is on the other side of the spacer bar. This outermost, or ‘secondary’, sealant is conventionally a two-part sealant based on one of polysulphide, polyurethane or silicone. Thermoplastic one-part sealants based on butyl rubbers, however, have also been used for this purpose, as have “reactive” hot-melt sealants which are applied as thermoplastic materials but later post-cured by the action of atmospheric moisture.
By way of background, it is to be understood that two-part sealants form a seal by virtue of a curing mechanism that begins on contact of the two parts, whereas non-thermoplastic one-part sealants form a seal by virtue of a curing mechanism that begins when the sealant is released into the environment from its storage container, and thermoplastic one-part sealants form a seal when the sealant cools from the molten state.
Returning to the ‘secondary’ sealant, the principle function of this sealant is to provide mechanical strength to hold the IG unit together and prevent rupture of the ‘primary’ sealant during the normal thermal cycling (i.e. expansion and contraction with temperature) that is experienced by the unit. As such the “secondary” sealant plays a major part in ensuring that the IS unit can pass European Standard tests EN1279-2 and EN1279-3. The secondary sealant may additionally act as a moisture vapour and/or gas barrier, further improving the performance and service life of the IG unit. The secondary sealant needs to be strong and flexible, with excellent adhesion to glass and spacer bar materials—typically anodised aluminium, stainless steel, or occasionally plastic are used as spacer bar materials.
The materials currently used for ‘secondary’ sealants, however, have several disadvantages.
Looking first at the known two-part polyurethane sealants, these often contain crude 4,4′-methylene diphenyl diisocyanate (MDI) within their curing agent. This is harmful to health, and therefore polyurethane sealants must be labelled as Harmful. As a result, extra care must be taken during handling and transportation, as well as with regard to disposal of empty curing agent drums. Consequently the cost of using polyurethane sealants is high. In addition, polyurethane sealants often contain a small amount of an organo-mercury compound as a curing catalyst. This is highly toxic if handled during manufacture, and further the use of organo-mercury compounds is currently under threat by legislation, for example in the Netherlands. Polyurethane sealants also typically involve the handling of moisture-sensitive materials during their production process, and therefore some materials (for example mineral fillers) need to be thoroughly dried as part of the manufacturing process. This involves the use of heat and a vacuum, which are both expensive. In addition, if the drying step is performed as an integrated part of the overall production process, the mix may need to be subsequently cooled before carrying on with the process. This costs further time and money. If the drying is inadequate, the cure speed of the final mixed sealant may be affected.
Considering now the known two-part polysulphide sealants, these typically contain manganese dioxide and thiram (bis(dimethylthiocarbanoyl)disulphide) within their curing agent, and again therefore these sealants must be labelled as Harmful. Further, grinding of the manganese dioxide, which is necessary for the manufacture of the curing agent, carries the possibility of causing a violent exotherm and hence is potentially dangerous. Yet further, polysulphide polymers are themselves harmful to aquatic organisms, and some polysulphide sealants also contain harmful solvents.
Turning to the known two-part silicone sealants, these are very expensive and have poor moisture vapour resistance and argon retention Consequently they are seldom used in the manufacture of domestic IG units.
Looking finally at the known one-part thermoplastic sealants based on butyl rubbers, these also have poor durability as compared with polyurethane or polysulphide systems. They are further expensive and require energy intensive heating systems for their application.
Given the above-described disadvantages of each of the conventional ‘secondary’ sealants used in IG units, there exists a need for an alternative ‘secondary’ sealant that is largely harmless to both the people manufacturing it and the environment, as well as unlikely to be restricted by impending legislation. Any new sealant must also be capable of being manufactured at a competitive price.